The present invention relates to an integrated circuit and, more particularly, to a hybrid integrated circuit, including both active and passive elements, and a novel package structure therefore. The present invention is intended to make a hybrid integrated circuit more resistant to the effects of repeated thermal expansion and contraction.
As a package structure for semiconductor integrated circuits or hybrid integrated circuits, an epoxy resin is widely used as a molding material, an insert-mold method is utilized which is suitable for mass production, and a circuit substrate and a lead frame are incorporated therein during the mold process.
FIGS. 1(a) and 1(b) show, respectively, a schematic cross section of a conventional hybrid integrated circuit and a plan view of a lead frame used therein. The lead frame 60 has a square support plate 61 centrally arranged, which sometimes is called a stage or tab. On the support plate 61 a circuit substrate 62 is mounted and is selectively connected to leads 63 by a wire bonding method. The above structure is finally molded as a package 64 using epoxy resin material. The circuit substrate 62 comprises a base member 66 made of, e.g., alumina ceramic, epoxy glass or silicon, and active and passive elements 67 disposed on the base member 66.
In such a package structure, differences in expansion coefficients, adhesion characteristics, etc. among the parts thereof can cause cracking problems in the mold package.
Usually, the expansion coefficient of the base member 66 is smaller than that of the molding material, and adhesive strength between the mold material and the support plate 61 of metal is lower than that of the mold material to the base member. When the hybrid integrated circuit (sometimes hereinafter called "device") is put into operation and subjected to repeated heat cycles of expansion and contraction, the lower portion of the mold package 64 cannot withstand the stress, which concentrates along a periphery of the support plate 61, and finally the mold package 64 of the device develops a crack 65 along a side or neighboring two sides of the square periphery of the support plate as shown in FIG. 1(a).
In order to relieve the stress on the support plate 61 due to an expansion difference relative to the mold material, it has been proposed in Japanese Unexamined Patent Publication No. SHO-52-95173 dated Aug. 10, 1977 by S. Kobayashi, to form the lead frame with an opening or void portion in the support plate. This publication discloses that the support plate of the lead frame is composed of a surrounding peripheral frame portion and either a single or a plurality of openings formed therein.
Another potential solution was disclosed in the Japanese Unexamined Patent Publication No. SHO-63-81966, dated Apr. 12, 1988 by H. Tsutani, et al. This publication discloses a device including a lead frame and a support plate having a plurality of cuts from the periphery toward the center of the support plate.
The above two disclosures are concerned with a semiconductor integrated circuit having a single monolithic semiconductor chip mounted on the support plate. Generally the semiconductor chip size (surface area) is small compared with the outline dimensions of the mold package. The hybrid integrated circuit including a circuit substrate and a plurality of active and passive elements mounted thereon is larger and tends toward higher integration, and thus the size of the circuit substrate occupies a substantial part of the mold package size. The stress generated in the mold package during operation, then, becomes a serious problem.